1. Field of the Invention
The present invention relates to a fuel cell including an electrolyte electrode assembly, and separators for sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. Each of the electrodes comprises an electrode catalyst and a porous carbon. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of a fuel cell (unit cell) for generating electricity. A predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field is formed on a surface of the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between adjacent surfaces of the separators such that a coolant flows along the separators.
Generally, fluid supply passages and fluid discharge passages extend through the fuel cell stack in the stacking direction of the separators. The fuel gas, the oxygen-containing gas, and the coolant flow into the fluid supply passages, and are supplied to the fuel gas flow field, the oxygen-containing gas flow field, and the coolant flow field, respectively. Then, the fuel gas, the oxygen-containing gas, and the coolant flow into the fluid discharge passages, respectively.
Specifically, U.S. Patent Application Publication US 2001/0019793 A1 discloses a unit of the fuel cell as shown in FIG. 12. A unit cell 3 includes a membrane electrode assembly 1 and collector plates 2 stacked on both surfaces of the membrane electrode assembly 1. The membrane electrode assembly 1 includes a pair of reaction electrodes 5, and an electrolyte membrane 4 interposed between the reaction electrodes 5.
At one horizontal end of the collector plate 2 in a direction indicated by an arrow X, an oxygen-containing gas supply passage 6a, a coolant supply passage 7a, and an oxygen-containing gas discharge passage 6b are arranged vertically. At the other horizontal end of the collector plate 2 in the direction indicated by the arrow X, a fuel gas supply passage 8a, a coolant discharge passage 7b, and a fuel gas discharge passage 8b are arranged vertically.
According to the disclosure of U.S. Patent Application Publication US 2001/0019793 A1, the coolant supply passage 7a is provided at a vertically middle position at one horizontal end of the collector plate 2. The coolant discharge passage 7b is provided at a vertically middle position at the other horizontal end of the collector plate 2. As shown in FIG. 13, a coolant flow field 9 is formed on a surface of the collector plate 2. The coolant flow field 9 comprises grooves extending horizontally for connecting the coolant supply passage 7a and the coolant discharge passage 7b. 
In the fuel cell system, when the coolant containing air is supplied into the coolant flow field 9 from the coolant supply passage 7a, or when the coolant is injected into the coolant flow field 9 just after assembling the fuel cell, the air in the coolant is likely to move upwardly in the coolant flow field 9, and the air in the coolant may be trapped in an upper region of the coolant flow field 9 undesirably. Therefore, the cooling function is not properly carried out in the upper region. The entire power generation surface of the unit cell 3 can not be cooled uniformly.